1. Field of the Invention
The present invention relates generally to memory device and, more particularly, to memory device with a phase-change memory cell.
2. Description of Related Art
Electrically writable and erasable phase-change materials have traditionally been used for memory devices. Phase-change materials, which may be formed of chalcogenide materials, can be electrically switched between two structural states of generally crystalline and generally amorphous local order. The generally crystalline state is a phase in which the material's atoms and/or electrons form a repeatable lattice structure, whereas the atoms and/or electrons of the generally amorphous state are randomly distributed. The structural state can also be switched among a range of detectable structural states of local order between the extremes of completely crystalline and completely amorphous states.
Currently favored chalcogenide materials that are used for phase change memory applications typically contain mixtures of Te, Se, Ge, Sb, Bi, Pb, Sn, As, S, Si, P, and/or O. Because of the range of structural states, a given as-deposited stoichiometric chalcogenide material can have varied bulk conductivities. Generally speaking, the more crystalline local order the state has, the higher the conductivity of the material. Moreover, the conductivity of the material can be selectively and repeatably established via an electrical pulse of given voltage and duration, herein called a setting or resetting voltage. The conductivity remains stable until another setting or resetting voltage of comparable size is applied. Furthermore, the conductivity of the material appears to vary inversely with the setting or resetting voltage and does not depend upon the previous state of the material, i.e., the material lacks hysteresis.
The aforementioned materials can be used to store and retrieve information within a non-volatile, overwritable memory cell. When different setting or resetting voltages are employed to change the conductivity of the material, the corresponding conductivities can be distinguished by various means including, but not limited to, the application of a relatively smaller voltage across the material within the cell. If, for example, two distinct setting or resetting voltages are used, one memory cell is able to store and retrieve one bit of binary encoded data. If more than two distinct setting or resetting voltages are used, then one memory cell is able to store and retrieve an analog form that can represent multiple bits of binary encoded data. Since the chalcogenide materials are able to maintain their respective conductivities, the memory cells are non-volatile, in that no refreshes are necessary to keep the data stored. The memory cells can also be directly overwritten, meaning that no data erasures are necessary prior to storing new data within the cells.
It is known that chalcogenide phase-change memory is not easy to incorporate into a CMOS circuit because the chalcogenide material requires a relatively high current density to change its state. Reducing the cross-sectional area of the chalcogenide part can reduce the current requirement in direct proportion. Structures which have been developed and which reduce this cross-sectional area involve fabricating ultra small contacts and depositing the chalcogenide into the contacts. One of the methods of fabricating ultra small contacts involves using a dielectric film, i.e., a spacer, to further reduce the photolithographic limit as referenced in U.S. Pat. No. 6,111,264. This technique can reduce the cross-sectional area, but the shrinking ratio is limited by the spacer thickness. For example, if the pore diameter is 1600 Å and the spacer thickness is 400 Å, the shrinkage area ratio is only about 4:1. The minimum pore diameter is determined by the photolithography and the spacer thickness. The shrinkage ratio can be limited. Thus, it can be difficult to scale down the chalcogenide parts in this fashion. If the chalcogenide parts cannot be scaled down, then relatively large current is required to cause a state change in the material. A requirement for larger current corresponds to a requirement for greater power to operate an array of such memory cells.
There can be additional problems once the pores are scaled down. For instance, the uniformity of the pore-to-pore diameters can be poor. Moreover, the small pores can place constraints on the chalcogenide deposition process since it will be more difficult to deposit materials into the tiny openings. For example, in the context of pores formed using the process of the preceding paragraph, overhang of the spacer may partially or fully occlude the pore, further compromising the reliability of the deposition procedure. If the bottoms of the pores receive poor bottom coverage, the electrodes beneath them may not be able to predictably change the phases of the chalcogenide parts. If the phases are not repeatable when a given current is applied, the memory cell cannot reliably store data. Another critical issue arises in aligning phase-change material with a contacting electrode. Because of the large current densities which may be involved, even relatively small misalignments may create large changes in current density which may adversely affect the ability to program phase-change memory cells.
A need thus exists in the prior art for a method of reliably aligning a contacting electrode with a phase-change memory element. A further need exists for a method of fabricating an electrode for making contact with chalcogenide material using a relatively small cross-sectional area.